This invention relates to structures and methods for detecting magnetic anomalies at remote locations and more particularly to an apparatus and method for using magnetometers and gradiometers of the superconducting quantum interference device (SQUID) type to detect a magnetic anomaly artificially created or otherwise near a borehole or the like.
It is desirable to magnetically log boreholes in order to determine the physical properties of rocks such as porosity and flow permeability. With sensitive magnetometers and gradiometers located in a deep borehole, on the order of two miles in depth, one can measure at significant distances magnetic fluctuations produced by below-ground and above-ground nuclear detonations.
At a great depth below the earth's surface, surface magnetic fluctuations and noise created by lightning, power generation, and other disturbances are greatly diminished thereby permitting the use of extremely sensitive magnetic detecting devices. The most sensitive of such devices known to the inventors are SQUIDs which require a safe, quiet cryogenic environment such as provided by the teachings of the above-cited U.S. patent application Ser. No. 202,041, now U.S. Pat. No. 4,340,405.
An important application of the present invention occurs in the development of hot dry rock (HDR) geothermal energy. To generate HDR geothermal energy, a deep (greater than 2,500 meters) borehole is drilled into an HDR zone and flooded under high pressure to create splits, cracks, or fissures caused by hydrofracturing. A second borehole is then drilled about two hundred (200) meters from the first so as to intersect the fracture zones. To generate geothermal power water is pumped down one borehole to flow through the cracks to the second borehole becoming heated by the HDR in the process and emerging as steam to run an electric power plant.
Because of the very great expense of deep borehole drilling, significant economies result if the azimuth of the hydrofracture crack system is known accurately without ambiguity prior to the drilling of the second borehole.